In vehicles, and especially for heavier vehicles such as trucks, a gearbox, also called range gearbox, is often connected the main gearbox to double the number of gears. Such an auxiliary gearbox usually includes a planetary gear, which has a low gear and a high gear, wherein the main gearbox shift facilities can be divided into a low range gear position and a high range gear position. In low range gear a downshift takes place through the planetary gear, and in the high range gear the gear ratio is 1:1 in the planetary gear.
The range gearbox is usually provided between the main gearbox and a propeller shaft coupled to the drive wheels of the vehicle. The range gearbox is accommodated in a gearbox housing and comprises an input shaft coupled to the main gearbox, an output shaft and between the input shaft and the output shaft is the planetary gear disposed. The planetary gear usually comprises three components, which are rotatable arranged relative to each other namely a sun gear, a planet carrier with planet gears and a ring gear. With knowledge of the number of teeth of the sun gear and the ring gear the relative speed of the three components can be determined during operation. In a range gearbox the sun gear can be rotatable connected to the input shaft, a number of planet gears which engage said sun gear, which planet gears are rotatable mounted on the planet carrier which is fixedly connected to the output shaft, and an axially displaceable ring gear which surrounds and engages the planet gears. The teeth of the sun gear, planet gears and ring gear can be helical, that is, they have an angle to a common rotation axis of the sun gear, planet carrier and ring gear. By cutting the teeth of the gears in an angle in relation to the rotation axis of the gear, the generated sound from the planetary gear is reduced. However, due to the helical teeth a reaction force arises from the gears in the planetary gear in the direction of the rotation axis. The direction of the reaction force depends on the direction in which the gears of the planetary gear are helical. Thus, the reaction force is directed backward or forward in the extension of the rotation axis.
In a known range gearbox the low range gear position and high range gear position are obtained by displacing the ring gear axially between the low range gear position, in which the ring gear is rotationally locked relative to the gearbox housing, and high range gear position in which the ring gear is rotatable relative to the gearbox housing and where the ring gear, the planet gears and the sun gear rotate as a common unity. The planetary gear comprises two on each side of the ring gear arranged coupling rings and two synchronizing rings arranged on each side of the ring gear. Synchronizer rings is to provide a synchronous shift.
In order to obtain a good synchronization function in this type of range gearbox, the surface of the synchronizer ring teeth, which faces the ring gear and which is intended to receive the ring gear teeth during synchronization, may be provided with an angle, called locking angle, relative to the axis of rotation of the synchronizer ring, which locking angle must be balanced against the braking torque the synchronizer ring transmits. This means that said locking angle must be so designed that the teeth of the synchronizing ring abuts on the part of the ring gear teeth which are provided with the locking angle and impact sufficiently on the ring gear so that synchronous speed can be obtained, and thereafter be detached from the part of the ring gear teeth, which are provided with locking angle, so that the ring gear can engage with the current coupling ring when the synchronous speed is obtained. To ensure that a synchronous speed is achieved before the ring wheel slip past the synchronizer ring in the axial direction, the teeth of the synchronizing ring must be so designed that they do not release the ring gear teeth too easy.
When the teeth of the synchronizing ring have been disengaged from the ring gear teeth and the synchronous speed has been obtained between the ring gear and the coupling ring, the ring gear will be further axially displaced so that the synchronizer ring is moved inwardly in the ring gear and stop in an axial position relative to the ring gear, which axial position is determined by the position where the synchronizing ring hits and abuts against the planet wheels of the planetary gear.
The freedom of movement in the axial direction of the ring gear is limited by the geometrical design of the ring gear and the teeth of the coupling ring. When the ring gear hits the axial end positions the tips of the teeth of the ring wheel abut a circumferential end face of each coupling ring. This means that the ring gear can no longer be displaced in the axial direction.
The document WO0155620 discloses a synchronization device in a planetary gear in which the planetary gear includes a sun gear, a planet carrier and a ring gear. The sun gear is rotatable connected with the input shaft and a number of planet gears engaging with the sun gear, which planet gears are rotatable mounted on the planet carrier, which is connected to the output shaft. An axially displaceable ring gear surrounds and meshes with the planet gears. Low and high gear are obtained by the ring gear being displaced axially between low range gear and high range gear.
These synchronization means are subject to wear and are costly to repair. If the range gearbox shall transmit large torque, these synchronization devices must exhibit considerable dimensions, which results in increased weight, increased space requirements and an increased moment of inertia. When the high range gear is engaged, the torque is transmitted from the sun gear to the planet gears, which means that facets may be formed on the tooth flanks of sun gear, which creates noise in the gearbox and accelerates the wear of the gears of the planetary gear.
There are range gearboxes in which the synchronization devices are replaced with coupling sleeves provided with splines. By controlling the transmission to synchronous speed between the two components to be assembled an axial displacement of the coupling sleeve along the two components is made possible in order to connect them. When the components should be detached the transmission is controlled so that torque balance occurs between the components so that the coupling sleeve is not transmitting torque. It then becomes possible to move the coupling sleeve along the components in order to disengage them from each other.
The torque balance is a condition where a torque acting on the ring gear, corresponding to the product of the torque acting on the planet carrier and the gear ratio of the planetary gear while a torque acts on the sun gear, corresponding to the product of the torque acting on the planet carrier and (1−[the gear ratio of the planetary gear]). In the case that two of the components of the planetary gear, sun gear, ring gear or planet carrier are coupled together by a coupling sleeve, the coupling sleeve transmits no torque between the planetary gear elements when torque balance occurs. Thus, the coupling sleeve can be easily displaced and the planetary gear components released.
U.S. Pat. No. 6,196,944 shows a planetary gear comprising a sun gear, a planet carrier with planet gears and a ring gear. The sun gear may be connected to the input shaft by means of a coupling sleeve in a low range gear position and disengaged from the input shaft in a high range gear position. In the high range gear position the input shaft is connected to of the planet carrier by means of the same coupling sleeve. The ring gear is firmly connected to a gearbox housing. The known planetary gear is arranged in an auxiliary gearbox, having only two gear positions.
The reverse gear in a transmission in a vehicle are often arranged in the main gearbox, which then comprises a gear which is engaged when the vehicle is to be driven in the reversed direction. The gear wheel, which is intended for the reverse gear, causes an elongation of the main gearbox, and an undesired increase in weight of the vehicle. The reverse gear wheel rotates in the opposite direction to the other gears in the main gearbox, which causes losses. Said gear wheel which is intended for the reverse gear has a tendency to produce undesirable noise in the transmission, which is a result of an intermediate gear wheel disposed between a lay shaft and a main shaft in the main gearbox.
U.S. Pat. No. 6,196,944 shows a gearbox for motor vehicles comprising a planetary gear comprising a first and a second sleeve acting on the planet carrier, the ring gear, the gearbox housing and the output shaft. The first and second sleeves are controlled as one connected unit.
The known transmissions often requires complete disassembly for replacement of a component, which means that it takes much time and it becomes costly to repair the gearbox.